Turbofan gas turbine engines for powering aircraft generally comprise inter alia a core engine, which drives a fan. The fan comprises a number of radially extending fan blades mounted on a fan rotor which is enclosed by a generally cylindrical fan casing.
There is a remote possibility with such engines that part or all of a fan blade could become detached from the remainder of the fan, for example as a result of the engine ingesting a bird or other foreign object.
It is known to provide the fan casing with a fan track liner which incorporates a containment system, designed to contain any released blades or associated debris. FIG. 1 shows a partial cross-section of such a fan track liner.
In the event of a “fan blade off” (FBO) event, the detached fan blade 8 travels radially outward, penetrating the attrition liner 10, septum 12 and aluminium honeycomb layer 14 until it reaches the metallic fan casing 16. The fan blade 8 then travels forwards where it is trapped by the hook 18. The fan track liner must therefore be relatively weak in order that any released blade or fragment thereof can penetrate sufficiently to be restrained axially whilst the liner distributes the load applied to the casing barrel as the radial restraint.
In addition to providing a blade containment system, the fan track liner includes an annular layer of abradable material which surrounds the fan blades. During operation of the engine, the fan blades cut a path into this abradable layer creating a seal against the fan casing and minimising leakage around the blade tips.
The fan track liner must also be resistant to ice impact loads. A rearward portion of the fan track liner is conventionally provided with an annular ice impact panel. This may typically be a glass-reinforced plastic moulding which may also be wrapped with GRP to increase its impact strength. Ice that forms on the fan blades is acted on by both centrifugal and airflow forces, which respectively cause it to move outwards and rearwards before being shed from the blades.
The geometry of a conventional fan blade is such that the ice is shed from the trailing edge of the blade, strikes the ice impact panel and is deflected without damaging the panel.
Swept fan blades are increasingly used in turbofan engines as they offer significant advantages in efficiency over conventional fan blades. Swept fan blades have a greater chord length at their central portion than conventional fan blades. This greater chordal length means that ice that forms on a swept fan blade follows the same rearward and outward path as on a conventional fan blade but may reach the radially outer tip of the blade before it reaches the trailing edge. It will therefore be shed from the blade tip and may strike the fan track liner forward of the ice impact panel.
A conventional fan track liner is generally not strong enough along its forward region to withstand ice impact and is therefore generally not suitable for use with swept fan blades. It is not possible simply to strengthen the fan track liner to accommodate ice impact, because this may disrupt the blade trajectory during an FBO event, and thereby compromise the operation of the fan casing containment system.
In recent years there has been a trend towards the use of thinner and lighter fan blades for performance. Thinner and lighter fan blades are more likely to buckle than to penetrate a fan track liner which has been optimised for swept fan blade ice impact durability. This may result in undesirable, forward ejection of high kinetic energy debris through the engine intake.
It is an objective of this invention to provide a gas turbine engine containment assembly that will substantially overcome the problems described above and that is suitable for use any fan blade design.